In the context of the present invention super-hard materials are defined as those materials having a Vickers hardness of no less than 2000 kg/mm2. These materials include a range of diamond materials, cubic boron nitride materials (cBN), sapphire, and composites comprising the aforementioned materials. For example, diamond materials include chemical vapour deposited (CVD) single crystal and polycrystalline synthetic diamond materials of a variety of grades, high pressure high temperature (HPHT) synthetic diamond materials of a variety of grades, natural diamond material, and diamond composite materials such as polycrystalline diamond which includes a metal binder phase (PCD) or silicon cemented diamond (ScD) which includes a silicon/silicon carbide binder phase.
In relation to the above, it should be noted that while super-hard materials are exceeding hard, they are generally very brittle and have low toughness. As such, these materials are notoriously difficult to cut. Any cutting method must be sufficiently aggressive to overcome the extreme hardness of the material to form a cut while at the same time must not impart a large degree of stress or thermal shock to the material which would cause macroscopic fracturing of the material due to its brittle nature and low toughness. As such, there is narrow operating window for achieving successful cutting of super-hard materials and many available cutting methods fall outside this operating window. For example, most cutting methods are not sufficiently aggressive to cut super-hard materials to any significant extent in reasonable time-frames. Conversely, more aggressive cutting techniques tend to impart too much stress and/or thermal shock to the super-hard material thus causing cracking and material failure. Furthermore, certain cutting methods have operational parameters which can be altered so as to move from a regime in which no significant cutting of a super-hard material is achieved into a regime in which cutting is achieved but with associated cracking and failure of the super-hard material. In this case, there may or may not be a transitional window of parameter space in which cutting can be achieved without cracking and failure of the super-hard material. The ability to operate within a suitable window of parameter space in which cutting can be achieved without cracking and failure of the super-hard material will depending on the cutting technique, the size of any transitional operating window for such a technique, and the level of operation parameter control which is possible to maintain cutting within the window of parameter space in which cutting can be achieved without cracking and failure of the super-hard material.
In light of the above, it will be appreciated that cutting of super-hard materials is not a simple process and although a significant body of research has been aimed at addressing this problem current cutting methods are still relatively time consuming and expensive, with cutting costs accounting for a significant proportion of the production costs of super-hard material products.
Super-hard materials are currently cut with one or more of:                (i) wire EDM (Electrical Discharge Machining) machines for electrically conducting materials such as doped CVD synthetic diamond, HPHT synthetic diamond, and cBN products;        (ii) high power lasers for electrically insulating materials such as un-doped CVD synthetic diamond, HPHT synthetic diamond and cBN products; or        (iii) cutting saws typically impregnated with other super-hard materials such as diamond.        
EDM cutting is efficient for electrically conductive materials, however cannot be used on any insulating materials. Traditional saws can be used for providing small cuts, but become time and cost inefficient when used for bulk processing, as well as struggling to provide good cuts at high depths. To obtain efficient cutting with lasers, the beams need to be focused to a small, very intense spot. Whilst a focused beam is very suitable for relatively thin products, the kerf losses, due to the fact the beam is divergent, having been focused down from a relatively large starting beam, result in a high amount of material wastage and increased laser cutting time when cutting thicker materials. This becomes a major problem when large, single crystals of CVD synthetic diamond or slugs of cBN need to be cut into relatively thin wafers.
Ideally, any method for cutting super-hard materials would provide a combination of the following features:                (i) a low kerf loss, e.g. using a highly collimated cutting beam;        (ii) a high cutting rate and reduced cutting time;        (iii) a high degree of flexibility such that the cutting technology can be applied to a range of super-hard materials;        (iv) a high degree of controllability to achieve precise control of cutting location, cutting velocity, cutting depth, and cutting width; and        (v) a low degree of material damage to achieve cutting of a super-hard material without causing damage such as cracking of the material.        
The aforementioned advantageous technical features must also be balanced against the economic viability of any cutting technique which is to be used in a commercial process. Economic viability will be dependent on:                (i) initial hardware costs; and        (ii) operating costs including:                    a. the cost of consumables such as power and gas supplies;            b. the cost of maintenance and the lifetime of the cutting apparatus which will be dependent to some extent on the complexity and reliability of the cutting apparatus;            c. operational cutting time versus down-time which will be dependent on the time required to set up the cutting apparatus between cutting operations; and            d. personnel costs, e.g. if skilled operators are required to run the apparatus which again will depend to some extent on the complexity and reliability of the cutting apparatus.                        
Given the above requirements for a commercial cutting process for super-hard materials, EDM cutting has become the industry standard for electrically conductive super-hard materials whereas high powered lasers have become the industry standard for cutting electrically insulating super-hard materials. While high powered lasers could also be used for cutting electrically conductive super-hard materials EDM cutting is often preferred due to reduced capital and running costs when compared to laser cutting.
The present inventors have re-visited the problem of cutting super-hard materials to assess whether any alternatives exists which may provide improved performance compared to the well accepted industry standards. In this regard, the present inventors have identified that electron beam cutting has the potential to out-perform high powered lasers. As previously described, high powered lasers suffer problems of kerf loss and relatively slow cutting speeds, particularly when cutting thick super-hard materials due to the application of a divergent beam. In contrast, electron beams can be made highly collimated with a very small spot size (i.e. a high brightness beam) and thus provide the potential for cutting with reduced kerf losses and potentially at higher cutting rates.
After initiating a research programme to assess the viability of electron beam cutting for super-hard materials, the present inventors found that electron beam cutting of super-hard materials had already been proposed back in the 1960's, prior to the widespread availability of high powered lasers and the establishment of lasers as the industry standard for cutting electrically insulating super-hard materials. For example, U.S. Pat. No. 3,417,222 filed in 1965 discloses an electron beam technique for cutting super-hard materials. Furthermore, U.S. Pat. No. 3,417,222 would appear to give some hints as to why electron beam techniques were not accepted as an industry standard and were ultimately superseded by high powered lasers as discussed below.
Prior to the availability of high powered lasers, methods for processing electrically insulating super-hard materials were generally limited to cleaving, sawing, grinding, and polishing. U.S. Pat. No. 3,417,222 discloses that electron beam cutting techniques were also devised for cutting super-hard materials such as diamond. U.S. Pat. No. 3,417,222 further discloses that electron beam cutting techniques involve complicated and expensive apparatus which limited the commercial acceptance of such techniques. For example, it is disclosed that a typical electron beam apparatus available at the time required the use of multi-chambered vacuum vessels and complex vacuum pumping systems. It is stated that such apparatus is costly and successful operation demands so much of the operator's attention to control the vacuum pumps that little time is available for observing and controlling the cutting operation.
In consequence of the above, U.S. Pat. No. 3,417,222 suggests the use of a more simple electron beam apparatus design using a single chamber vacuum system serviced by a single vacuum pumping system. However, the present inventors note that any such vacuum system adds a level of complexity and cost which is not required by laser technology and is presumably the key reason why laser technology appears to have entirely superseded electron beam technology for cutting super-hard materials such as diamond. In this regard, it may be noted that electron beam cutting was never widely accepted as a suitable choice for commercial cutting of super-hard materials and laser cutting has become so ubiquitous that many skilled persons in the art of super-hard materials cutting at the time of writing this specification would be unaware of the early work relating to the use of electron beam cutting for super-hard materials.
Despite the above, it would appear that U.S. Pat. No. 3,417,222 suggests some reasonably good cutting performance using their electron beam cutting technique. The apparatus described in U.S. Pat. No. 3,417,222 is configured to provide a focussed electron beam at a point on a material sample held in a specially adapted vice mounted on a movable table. Cutting is achieved by moving the table such that the material sample is moved relative to the electron beam. A gas lance is provided for directing a gas stream onto the material sample at the focus point of the electron beam. It is described that the gas stream improves the quality of the cut and the cutting rate. An example is given for cutting diamond material using an oxygen gas stream. It is stated that providing a stream of oxygen at the focus point of the electron beam can increase the cutting rate of diamond by a factor of 5 and suggests that a 1-carat gem can be cut in about 10 minutes using this technique rather than a time as long as 8 hours required using conventional sawing. Given that a factor of 5 improvement in cutting speed is reported for the electron beam cutting technique using an oxygen gas flow when compared with a comparable electron beam cutting technique without an oxygen gas flow, this would suggest that a 1-carat gem can be cut in about 50 minutes using the technique without oxygen gas flow. In this regard, it is interesting to note that an equivalent cutting process using a high powered laser would take of the order of 40 minutes which is intermediate between the two electron beam cutting times with or without the use of an oxygen gas stream.
In light of the above, it would appear that the electron beam cutting technique using an oxygen gas stream is actually slightly quicker than a high powered laser technique although still of the same order of 10's of minutes. It is supposed that this apparent cutting rate advantage for the electron beam technique when compared with high powered lasers was not sufficient to off-set the higher costs and complexity associated with the electron beam cutting technique and is why laser technology appears to have entirely superseded electron beam technology for cutting super-hard materials such as diamond.
U.S. Pat. No. 3,417,222 discloses cutting diamond with an electron beam at an acceleration voltage of 130 kV. It is also disclosed that by using an oxygen gas stream the electron beam current can be reduced from 10 mA to less than 1 mA. It is also disclosed that in the absence of the oxygen gas the electron beam must be employed at a slow pulse rate in order to minimize crystal breakage during cutting while the addition of an oxygen gas stream enables a substantially continuous smooth cutting by pulsing the electron beam at a very rapid rate of about 35 counts per second thus increasing cutting rate.
U.S. Pat. No. 3,417,222 thus appears to recognize that crystal breakage is a problem when using an electron beam cutting technique and proposes to use a pulsed electron beam having a low current and a gas stream directed at the electron beam focal point on the material being cut in order to alleviate this problem.
The present inventors have also found that cracking and crystal breakage is problematic when using an electron beam cutting technique for super-hard materials such as diamond. In fact, several years prior to the present invention the present inventors trialed electron beam cutting for super-hard materials and discounted the technique for this reason. As previously mentioned, while super-hard materials are exceeding hard, they are generally very brittle and have low toughness. As such, electron beam cutting has been dismissed in the past as being a suitable choice for cutting super-hard materials. Furthermore, while using a pulsed electron beam having a low current and a gas stream as described in U.S. Pat. No. 3,417,222 may alleviate these problems, the use of a low current electron beam significantly reduces cutting rates to a similar order of magnitude to that achievable using more simple, lower cost laser systems.
In contrast to the above, the present inventors have developed a new electron beam cutting technique for super-hard materials which provides a cutting time improvement of over an order of magnitude (even up to two or three orders of magnitude for certain super-hard materials) when compared with both the electron beam cutting technique described in U.S. Pat. No. 3,417,222 and current high powered lasers, thus more than off-setting the higher costs and complexity associated with an electron beam cutting technique. A summary of the new electron beam cutting technique and how it differs from that described in U.S. Pat. No. 3,417,222 is given in the following summary of invention section with detailed embodiments set out thereafter in the detailed description.